Composite materials, such as those that include fiber and/or organic resin, are characterized by strength, low weight, and weather resistance, making them attractive substitutes for traditional materials in many applications. For example, fiber reinforced polymer (FRP) matrix composites are replacing conventional metallic materials and becoming increasingly popular engineering materials in many sectors of industry, such as aircraft, wind blades, automobiles, naval construction, infrastructures, and offshore structures. However, these composite materials may also display undesirable characteristics upon exposure to open flame or high levels of radiant heat, including surface flammability, smoke generation, and toxic product generation. Because heavy smoke hinders escape efforts and toxic fumes may cause the death of occupants, use of such composites in the construction of buildings, aircraft, watercraft, and vehicles has been limited. Thus, though metal materials are undesirably heavier and corrosive, the flammability of composite materials prevents their substitution in many applications.
Various approaches have been proposed to reduce the flammability of composite materials. One method involves combining flame resistant additives with polymer matrices. In FRP resins, for example, a flame resistant additive may be added to the resin prior to fiber impregnation. However, these additives also display problems such as poor compatibility, leaching, and a reduction in strength and/or other desired mechanical properties of the composite material. For example, traditional halogen-based flame retardant additives are extremely effective at reducing flammability but may release corrosive and toxic chemicals in use.
Another proposed method involves using nanoscale fillers such as SiO2, Al2O3, TiO2, layered silicates, graphite, or carbon nanotubes to improve the flame retardancy of polymers. When exposed to a flame, a network of carbon nanotubes within a material will act as a barrier for chemical and thermal transport while also providing a protective char on the material surface. The surface char decreases the emission of toxic chemicals and the displacement of oxygen and also provides a thermal barrier that decreases the rate of heat dispersion, thereby minimizing the spread of the flame. Further, such nanofillers may not produce toxic gases. However, strong van der Waals forces between the nanofillers make uniform dispersion of the fillers in a matrix material difficult, and the addition of the fillers significantly increases the viscosity of polymer resin, which may in turn create processibility problems. In fiber-reinforced composites, for example, the flow of resin through the porous fiber mats is difficult and the fiber mats undesirably may filter out the fillers during the manufacturing process. Research has also indicated that nanocomposite material failures are primarily due to poor particle dispersion. Homogenous filler dispersion is thus an obstacle to implementing the use of nanoscale fillers for improving fire retardancy.
U.S. Patent Application Publication No. 2009/0148637 by Zhang, et al. discloses manufacturing composites or nanocomposites with carbon nanotube membranes for flame resistant applications. Specifically, free-standing mats (buckypapers) of entangled carbon nanotubes or nanotube ropes are proposed as flame retardant shields on the surface of composites. When exposed to a flame, a buckypaper will act as an excellent insulator, protecting the underlying polymer composite material. However, the buckypaper fabrication process can be time-consuming and expensive.
It would therefore be desirable to provide improved fire retardant materials and easier, more cost-effective fabrication methods, to reduce or avoid some or all of the foregoing deficiencies and limitations.